1. Field of the Invention
The invention relates in general to transport refrigeration systems, and more specifically to new and improved temperature controllers, and methods of controlling the load temperature of such refrigeration systems.
2. Description of the Prior Art
Transport refrigeration systems for trailers and trucks have many different operating control arrangements for improving load temperature control while reducing fuel costs of the internal combustion engine, such as a Diesel engine, which commonly drives the refrigeration compressor. For example, it is common to employ a plurality of different heating and cooling modes, depending upon where the load temperature is relative to the set point temperature. In a cooling mode, in which the load requires cooling to maintain the set point, the hot gas output from the compressor is directed through a first fluid circuit which includes a condenser, expansion valve, and evaporator. In a heating mode, in which the load requires heating to maintain the set point, or in which the evaporator requires defrosting, the hot gas is directed through a second fluid circuit which by-passes the condenser and goes directly to an evaporator defrost pan heater and then through the evaporator.
To provide different heating and cooling capacities, it is common to operate the compressor at one of two different speeds, called high speed and low speed. Still additional capacity control may be provided by operating the compressor partially unloaded during the low speed heating and cooling modes.
In an effort to conserve fuel, when a shift from low speed to high speed is called for by the load temperature control, it is known to delay the change from low to high speed to insure that the requirement is not transitory. If the system is still calling for high speed heating or cooling after the time delay, then the shift to high speed is made. U.S. Pat. No. 4,325,224, which is assigned to the same assignee as the present application, sets forth examples of different heating and cooling modes, including partial heating and cooling modes via compressor unloading, and the use of time delays between low and high speed modes.
In a further effort to conserve fuel, instead of operating the compressor continuously, there are times when the compressor and its driving engine may be turned off for significant periods of time without deleteriously affecting load temperature. U.S. Pat. No. 4,419,866, which is assigned to the same assignee as the present application, discloses the alternative use of either continuous or start-stop compressor operating modes.
My co-pending application Ser. No. 020,259 filed Feb. 27, 1987, entitled "Temperature Controller For A Transport Refrigeration System", now U.S. Pat. No. 4,819,441 which is assigned to the same assignee as the present application, discloses a new temperature controller for transport refrigeration systems in which an analog ratio between the load temperature and the set point temperature is developed. The analog ratio is converted to a digital signal in a digital to analog converter, including scaling the converter inputs such that the set point is at the midpoint of the possible number of bit changes from all logic zeros to all logic ones. Further, the scaling and the number of bits in the digital signal are selected such that all logic zeros and all logic ones represent implausible temperature deviations from the set point. Thus, a digital signal in which all the bits are logic zero or logic one may conveniently be used to provide a sensor failure signal. Low cost programmable logic arrays (PAL'S) are programmed to utilize different values of the digital signal to provide a plurality of different control signals which may initiate any desired number of different heating and cooling modes, as well as recognize events which may be used to start external timers. The timers then provide signals for the PAL which initiate, maintain, or terminate, predetermined heating and cooling modes, as desired.
While the temperature controller of my co-pending application performs well, it would be desirable, and it is the object of the present invention, to provide a new and improved temperature controller for transport refrigeration systems, which retains the digital, programmable aspects of the controller in my co-pending application while reducing the complexity and cost of the controller. It would further be desirable to increase the resolution of the controller.